Silicon has been in a unique position in the semiconductor industry due largely to the ease of making a passive, water-insoluble masking film and growing or depositing an oxide film on its surface. The nature of the interface region in the Si/SiO.sub.2 systems show large variation depending on the processing techniques; see for Example: The Physics and Chemistry of SiO.sub.2 and SiO.sub.2 Interface. Helms and Deal (eds), Plenum Press, New York (1988). Adverse interface properties, such as a large number of charge trap centers, can alter or hinder the electrical behavior of the oxide-semiconductor structure. Commercial interest in detecting and eliminating these irregularities stimulated intense research efforts to understand and improve the quality of the interface properties. In particular it has been observed that the density of the interface traps can be altered by orders of magnitude by a low-temperature annealing of the sample in different ambient gases. Even though the process by which the trap centers are activated or passivated are not well understood, it is definitely affected by hydrogen, thought to be atomic in nature.
Positron annihilation studies of solid materials have grown rapidly in recent years because of the advances in the development of relatively intense, low-energy positron beams, as reviewed by Schultz and Lynn, Rev Mod. Phys., 60, 701 (1988). These studies utilize the fact that the positron annihilation process is entirely decided by the initial state of the positron-many electron system. When positrons are implanted in a solid they are rapidly thermalized (.about.1-10 ps), and these positrons annihilate with electrons either directly or after forming a bound system known as positronium. In the case of direct annihilation two 511 keV .gamma.-rays are produced, which are broadened by the motion of the electrons and positrons prior to the annihilation. Since the thermalized positrons have much lower momentum compared to electrons in the solid, the Doppler broadening of the 511 keV .gamma.-spectrum will depend mostly on the electron motion. Hence the annihilation spectra have been used to extract information about the electron environment around the annihilation site. As positrons are increasingly employed as a probe to study surface properties, interface structures and defect profiles, a proper prescription of the implantation profile and means for data analysis is needed.
Variable energy positron beams have recently been applied to the study of SiO.sub.2 /Si structures and has shown new results because of the unique depth and defects information they can provide. See, for Example, Nielsen, Lynn et al., Appl. Phys. Lett., 51, 1022 (1987); Nielsen, Lynn et al. Phys. Rev., B40, 1434 (1989); Lynn, Nielsen et al., Can. J. Phys., 67, (1989) and Uedono et al., Phys. Lett. 133A, 83 (1988). Existing variable-energy positron beam experiments have exclusively been measuring the Doppler broadening by characterizing the annihilation lineshape with a lineshape parameter known as S-parameter; see Schultz and Lynn (1988), supra. The S-parameter technique has been applied in several detailed studies like SiO.sub.2 /Si interface, see Nielsen, Lynn et al. (1987) supra., and Nielsen, Lynn et al. (1989) supra.; to the hydrogen-sensitive interfacial defects, see Lynn, Nielsen et al. (1989) Supra.; and to electric field effect at the Si surface, Uedono, Tanagawa et al. (1988) supra.
For many years various experimental techniques have been used to identify and understand the properties of the interface and its trapping centers, for Example capacitance-voltage measurements (CV), electron spin resonance (ESR), transmission electron microscopy (TEM), etc. Most of these techniques, however, are either destructive or requires special preparation. In some cases the measurement process itself permanently alters the electrical properties of the SiO.sub.2 /Si wafer. Therefore, it would be desirable to devise a non-destructive and depth resolving technique, which could probe semiconductor devices, such as MOS, for quality control, research and testing in the semiconductor industry.